Generally speaking, a safety or pressure relief valve comprises, on one hand, a hollow body closed by a plug defining a revolution volume formed from a relatively large central chamber and two relatively narrow opposed chambers and, on the other, a floating piston with its ends engaged in said chambers, one side of this piston being subjected to a reference thrust and the other to the pressure of the fluid to be controlled, the central chamber being in communication with the outside by a release hole.
Such a pressure relief valve is described in the German Pat. No. 1.083.096 (Eddelbutteb 1960) and the French Pat. No. 1.384.817 (SITA-1963).
Such valves suit the canalizations conveying non abrasive fluids subjected to medium pressure.
In the case of high pressure abrasive fluids (mud or cement, of the oil industry, under 700 bars, for example), at the moment the pressure to be controlled undergoes a sudden increase and generates, on the control end of the piston, a force greater than the reference thrust force applied to the other end, the fluid under control escapes at great speed all around the control end of the piston and its pressure drops immediately by lamination.
Due to the solid particles carried at great speed by the fluid, the edges of the chamber under control and those of the concerned end of the piston are rapidly eroded, with the result that, after a small number of discharges, a loss of sealing capacity is produced, at this level, which renders the valve unusable as is and requires it to be repaired.